Simplified, empirical model of wind speed profile under canopy of Istebna spruce stand in mountain valley

Abstract

The wind speed in a forest is one of the main factors deciding upon the amount of water balance between the atmosphere and the soil. Inside a forest complex, the distribution of vertical wind speed has a characteristic S-shape. This paper presents a simplified, empirical model of such an S-shaped wind profile within a stem layer of an Istebna spruce stand. Measurements of the wind speed were taken at four levels in 9 homogeneous, even-aged stands of different age-class: from a 10-year-old natural seeding to a 121-year-old mature stand. Recorded data unambiguously confirmed the existence of the secondary maximum inside the stem layer. The main aim of this paper was to make all model parameters conditional on biometrics features of tree stand as well as the geographical location of research sites on the slopes in a mountain valley. All parameters describing the simplified, empirical model of S-shaped wind profile within the stem layer strictly depend on the well-known total basal area TBA [m2h−1]. TBA is determined by only an average number of trees per hectare and a mean diameter at a breast height, i.e. 130cm above ground, which are easy to be measured with high accuracy ‘from ground’. Presented data analysis shows that the estimated wind speed at the secondary maximum and a mean wind speed within a stem layer are contingent upon a biomass density in the crown layer and its elevation above ground. A biomass density can be equated with the leaf area index (LAI).The presented simplified, empirical model of the vertical wind speed profile for a spruce stand and its parameterization scheme, which is to be measured in an effortless way, should be easily adaptable to stands composed of other tree species, taking into consideration specific differences between spaces. The necessary improvements of the presented empirical model would be possible after some series of measurements taken within various types of tree stands. A thorough understanding of the biological factors creating the observed S-shaped wind profile ought to considerably expand the knowledge of the forest ‘ventilation’ process, which determines vaporization from the soil as well as the CO2 advection.